Wind turbine with oscillation damping means

ABSTRACT

The present invention relates to a wind turbine with oscillation damping means provided at the nacelle and being designed for damping edgewise oscillations of the rotor blades in the rotational plane of the rotor. In particular, the invention relates to a wind turbine in which the oscillation damping means are provided at the end of the nacelle being opposite to the end from which the rotor extends and are designed for damping oscillations of the first eigenfrequency of the rotor blades in the rotational plane, especially oscillations being perpendicular to the rotational axis of the rotor. The damping means are advantageously designed to dampen oscillations of a frequency being substantially equal to the first eigenfrequency in the rotational plane of the at least one blade minus the frequency of rotation of the rotor. The oscillation damping means of the present invention are preferably capable of damping oscillations being substantially horizontal and substantially perpendicular to the rotation axis.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/DK99/00289 which has an Internationalfiling date of May 31, 1999, which designated the United States ofAmerica.

The present invention relates to a wind turbine with oscillation dampingmeans provided at the nacelle and being designed for damping edgewiseoscillations of the rotor blades in the rotational plane of the rotor.

In particular, the invention relates to a wind turbine in which theoscillation damping means are provided at the end of the nacelle beingopposite to the end from which the rotor extends and are designed fordamping oscillations of the first eigenfrequency of the rotor blades inthe rotational plane, especially oscillations being perpendicular to therotational axis of the rotor.

BACKGROUND

Undesired oscillations may occur in wind turbines for power generation.These oscillations may include the entire turbine, i.e. several partsare oscillating in a combined mode, or the oscillations may occurlocally in single part. Of particular severity are oscillations in therotor blades either edgewise (in the rotor plane), flapwise(perpendicular to the rotor plane), or in a combined edge- and flapwisemode. Whether these oscillations do occur is dependent on the windturbine design and the meteorological conditions.

Blade oscillations may be dampened by building a damping device into theblades, such as described in WO-A-95/21327, but it is difficult toproduce a feasible design that is sufficiently compact and flat in orderto satisfy the severe spatial restrictions. Furthermore, to build in adamper into existing blades is difficult and expensive.

The oscillation phenomena may cause dangerously high loads on the bladeand other parts of the wind turbine, which may lead to a sudden collapseor alternatively may be the cause of fatigue damage and lifetimereduction, as cracks in the components slowly grow ultimately leading tofailure. The occurrence of oscillations adds an uncertainty factor topredictions of lifetime loads on the various parts of a wind turbine,making it necessary to make the design stronger and heavier than wouldotherwise be the case.

DESCRIPTION OF THE INVENTION

Oscillations of the blades in the rotational plane of the rotor, theedgewise oscillations, are particular severe and may cause suddenfracture of the base part of the blades and may under unfortunatecircumstances cause blades to break off the rotor.

The edgewise oscillations of the blades cause the centre of gravity ofthe rotor to oscillate, and the oscillations are transmitted to thenacelle on which the rotor is arranged. An alternative to dampening ofthe oscillations by arranging damping means in the blades have beenfound to be dampening of the oscillations of the nacelle. By arrangingoscillation damping means at the nacelle for damping oscillations of anappropriate frequency the oscillations of the blades may be dampened toa harmless level since the nacelle and the rotor oscillates together.

This technical solution of the problems of edgewise blade oscillationsis advantageous because it may be installed in existing wind turbinescontrary to dampers in the blades, it is an inexpensive solution and anoscillation damping means in the nacelle may easily be accessed foradjustment and maintenance.

It is an object of the present invention to provide dampening ofedgewise oscillations of the blades of a wind turbine by oscillationdamping means arranged at the nacelle of the wind turbine.

Thus, the present invention relates to a wind turbine comprising

a foundation,

a tower extending substantially vertically and being mounted at a lowerend on the foundation,

a nacelle supported by an upper end of the tower,

a wind rotor having at least one blade arranged on a main shaft having asubstantially horizontal rotation axis and being arranged at the nacellewith the wind rotor extending from one end of the nacelle, the at leastone blade defining a rotational plane being perpendicular to therotation axis, and

oscillation damping means provided at the nacelle and being designed fordamping oscillations of the at least one blade in the rotational plane.

The oscillation damping means may be arranged inside the nacelle or onthe outside of the nacelle, mainly depending on the spatial restrictionsinside the nacelle. However, it is for most wind turbines advantageousthat the oscillation damping means are arranged at the end of thenacelle being opposite to the end from which the rotor extends becausethe nacelle is arranged pivotally on the tower about a central verticalyawing axis and the amplitude of the horizontal oscillations of thenacelle increases with the horizontal distance to the yawing axis forwhich reason oscillation damping means are more efficient at thisposition. Alternatively, the oscillation damping means could be arrangedanywhere near the periphery of the nacelle, even at the end at which therotor extends from the nacelle.

The oscillation damping means should be capable of at least dampingoscillations being substantially horizontal and substantiallyperpendicular to the rotation axis since the oscillations of the rotorcaused by the edgewise oscillations of the blade(s) are perpendicular tothe rotation axis. The vertical oscillations are normally dampenedsufficiently by the stiffness of the vertical connection between thenacelle and the tower, whereas the horizontal oscillations are lessdampened because the yawing arrangement commonly has a certainclearance.

The oscillation damping means are preferably designed for dampingoscillations of a frequency being substantially equal to the firsteigenfrequency in the rotational plane of the at least one blade minusthe frequency of rotation of the rotor. This mode has in most cases byexperiment and simulations proven to be the frequency having the largestamplitude. However, for some construction it has be found to beadvantageous to, alternatively or additionally to this frequency, todesign the oscillation damping means for damping oscillations of afrequency being substantially equal to the first eigenfrequency in therotational plane of the at least one blade plus the frequency ofrotation of the rotor.

In case both of the aforementioned frequencies should be dampened, afavourable embodiment of the oscillation damping means comprises firstdamping means designed for damping oscillations of a frequency beingsubstantially equal to the first eigenfrequency in the rotational planeof the at least one blade minus the frequency of rotation of the rotorand second damping means connected to an oscillating mass element of thefirst damping means and being designed for damping oscillations of afrequency being substantially equal to the first eigenfrequency in therotational plane of the at least one blade plus the frequency ofrotation of the rotor The second damping means may with such anarrangement be design much smaller that the first damping means, theoscillating mass of the second damping means being typically of about atenth of the oscillating mass of the first damping means.

The oscillation damping means are in preferred embodiments designed fordamping oscillations of a frequency deviating less than 0.2 Hz,preferably less than 0.1 Hz and most preferred less than 0.03 Hz fromthe first eigenfrequency in the rotational plane of the at least oneblade minus, respectively plus, the frequency of rotation of the rotor.

The frequency of the oscillations that the oscillation damping means aredesigned for damping may be of variable frequency and comprise means forvarying said frequency at least according to variations of the frequencyof rotation of the rotor. Such means could be means for varying the massof a mass element by pumping a liquid such a water to and from a liquidcontainer arranged on or constituting the mass element.

The oscillation damping means are suitably designed for dampingoscillations of a frequency in the range of 0.5-4 Hz, preferably in therange of 1-3 Hz.

The oscillation damping means should be designed for dampingoscillations of the at least one blade in the rotational plane so thatthe oscillations at all operating conditions are dampened to a harmlessamplitude level. This may be obtained with a damping of a magnitudebeing equivalent to a logarithmic decrement of oscillations of the firsteigenfrequency in the blade(s) comprised within the rotor of preferablyat least 1.5% and most preferred of at least 2.5%. The logarithmicdecrement of oscillations of the first eigenfrequency in the blade(s) istypically of an order of 2-4% and the maximum aerodynamic excitation ofthe blades is typically of an order or 5-7%. The sum of the logarithmicdecrement of oscillations of the first eigenfrequency in the blade(s)and the damping by means of the oscillation damping means should besufficient to counteract the aerodynamic excitation. On the other hand,the damping by means of the oscillation damping means should not be tooexcessive to ensure that the oscillation damping means react to andcounteract oscillations of minor amplitudes which may cause fatigue ofparts of the wind turbine. The above-mentioned sum should therefore notbe much higher than the aerodynamic excitation of the blade(s) inquestion and typically of an order of 6-8%.

The oscillation damping means may according to the present invention becapable of damping oscillations in more than one direction. Thedirection may be controlled by adjusting the damping means, such as,e.g., by turning the damping means about a vertical axis, or theoscillation damping means may be capable of damping oscillations in morethan one direction simultaneously. Furthermore, the oscillation dampingmeans may be constructed in a symmetric or asymmetric configuration,exhibiting different or identical resonance frequencies in two or moredifferent directions.

It is in general preferred to employ oscillation damping meanscomprising at least one mass element and/or at least one spring elementand/or at least one damper element. The oscillation damping means may beof a variety of configurations, such as comprising

a directional movable mass element connected to the nacelle by a helicalspring element and a shock absorber,

a pendulous mass element pivotally mounted in one end of one or morearms, the other end of said arm or arms being pivotally mounted to aframe, where the assembly points between the arms and the frame and/orthe mass element are provided with resilient elements,

a mass element supported by one or more resilient columns, preferablyconsisting of one or more steel members combined with one or moremembers made of plastic, rubber or the like in a layered construction,such as a sandwich construction,

a mass element arranged as a pendulum connected to an input shaft of agearbox having an output shaft connected to a hydro-coupling,

an elongated mass element of a magnetic material arranged inside a coilelectrically connected to a resistor, said mass element being mounted tothe nacelle, preferably over a helical spring,

a liquid container mounted to the nacelle, where the liquid in saidcontainer constitutes the mass element, the damper element and thespring element,

a pendulous mass element suspended in a suspension member, such as arubber pipe, or

a container with a concave bottom surface provided with one or moretoothed bars that are in engagement with at least one gear wheelcylindrical mass element that can roll along the bottom surface, saidcontainer being at least partly filled with a damping material, such asa liquid.

In a preferred embodiment of the present invention, the oscillationdamping means comprise at least one helical spring connected to thenacelle at a first end and connected to a mass element at the other end,the mass element being supported vertically by at least one wheel memberwhich by its rim engages a damper element being a resilient material. Ina further preferred embodiment, the oscillation damping means compriseat least two helical springs connected to the nacelle each at a firstend and extending in substantially opposite directions from the firstends, the other end of each spring being connected to a common masselement so that the two springs are in a pre-tensioned state.

The wind turbine according to the invention may for safety, formonitoring as well as for control purposes comprises

an oscillation sensor for detecting oscillations of the rotor andproducing an output accordingly,

a control unit for receiving the output from the oscillation sensor andhaving means for determining, by use of the output, whether a predefinedsafety condition is fulfilled and means for regulate the operation ofthe wind turbine in case the safety condition is fulfilled, and

data storage means for storing data being significant of the output ofat least a time period previous to a moment of fulfilment of saidcondition.

The oscillation sensor, such as an accelerometer, may according to theinvention be used to evaluate the performance of the oscillation dampingmeans of a wind turbine by outputting the stored data being significantof the output of the sensor for a time period immediately previous tothe moment where the condition was fulfilled and analysing said data.The oscillation damping means may by use of the method be evaluated in asituation in which it did not perform satisfactory, e.g. in which theamplitude of the oscillations reached an unsatisfactory level and theoperation of the wind turbine was stopped, and its performance may beanalysed from the data of the last time period, such as, e.g., the lastminute or so before the moment of fulfilment of the safety condition.The oscillation damping means may be adjusted as a result of theanalysis, e.g. by adjustment of the mass of the mass element of theoscillation damping means.

The invention also regards a method of damping oscillations at least ina rotational plane of a wind turbine rotor comprising at least one bladeby means of applying oscillation damping means at the nacelle accordingto any the above disclosed embodiments.

The invention is in the following explained in more detail withreference to the accompanying drawings, in which:

FIG. 1 shows a wind turbine according to the invention,

FIG. 2 shows a side view of the nacelle of a wind turbine according tothe invention,

FIG. 3 shows a top view of the nacelle,

FIG. 4 shows a schematic view of a two directional damping deviceaccording to a preferred embodiment of the invention,

FIGS. 5-13 shows various preferred embodiments of a damping deviceaccording to the invention, and

FIGS. 14-15 shows the most preferred embodiment of a damping deviceaccording to the invention.

FIG. 1 shows a wind turbine having a rotor 1 comprising a number ofblades 2. The rotor 1 is arranged at a nacelle 3 placed on the top of atower 4. The wind turbine according to the invention comprisesoscillation damping means 5 at one or more of the positions A-F.

In FIGS. 2 and 3 two positions of the damping device 5 are shown. In apreferred embodiment of a wind turbine according to the invention thedamping device 5 is placed in the end of the nacelle 3 opposite therotor 1. The damping device is shown in more detail in FIG. 5. It ispositioned with the mass element m1 downwind from the rotor. In additionto or as an alternative to the position in the nacelle 3 the dampingdevice can also be placed in the tower.

In FIG. 4 is shown a damping device having a mass M suspended in twospring elements S and two damping elements D.

In FIG. 5 a tuned damping device according to one of the preferredembodiments of the invention is shown. A mass element m1 is arranged ina position where it is movable sideways. It is connected to the frame ofthe nacelle (or the tower) via a spring s1 and a shock absorber d1arranged parallel to the spring s1.

In FIG. 6 another embodiment is shown, where the mass element m2 canswing from side to side. It is suspended in to the frame 6 via two arms7. The arms 7 are connected to the frame and/or to the mass m2 via anessentially square rod at the mass m2 or the frame 6 and an oversizedessentially square mounting opening in the ends of the arms 7. Betweenan mounting opening in the arms 7 and the square rod of the massm2/frame 6 a number of fittings ds2 made from a resilient material arearranged.

In the embodiment of FIG. 7 the mass element m3 is placed on two columnsds3, that are made of a rubber or plastic core in a steel shell(sandwich construction).

In FIG. 8 the mass element m4 is pendulously arranged and connected tothe input shaft of a gearbox 8. The output shaft of the gearbox isconnected to a hydro-coupling d4, whereby the pendulating mass m4 isdamped.

In FIG. 9 an electrical damping device is shown. When the magnetic (ornon-magnetic) mass element m5 is shifted from side to side a current isgenerated in the coil 9. The coil is connected to a resistance 10. Bychoosing the size of the resistance 10 the amount of damping can beregulated.

In FIG. 10 an embodiment is shown, where a liquid container 11 is placedon the frame 6. The liquid mds6 in the container 11 served as both mass,damper and spring element in the tuned damping device.

In FIG. 11 a variation of the embodiment of FIG. 10 is shown where thecontainer bottom is concave and fitted with a toothed bar 14 at the twoopposite ends of the container bottom. A cylindrical mass element m8 isarranged in the container 12. The mass element m8 is provided with agear wheel s8 at each end, that are in meshing contact with acorresponding toothed bar 14. The mass m8 can roll along the bottom 13of the tank 12. The container 12 is at least partly filled with adamping liquid d8.

In FIG. 12 a mass element m7 is hanging in a rubber pipe ds7 servingboth as a damping and spring means of the tuned damper according to theinvention.

In FIG. 13 an embodiment is shown which consists of a conventionalmass-helical spring system, as indicated by m9 and s9. Damping action isprovided by deformation of the rubber d9, which may be placed on thewheels 14, on the rails 15, or both.

A most preferred embodiment of the oscillation damping device is shownin FIG. 14, as seen from above, and in FIG. 15 in a side view. Thedevice comprises four helical springs 16-19 arranged in parallel in adirection being perpendicular to the rotation axis of the rotor. Each ofthe four springs 16-19 is fixed to a frame part of the nacelle at afirst end 20-23 and is fixed to a mass-carrying frame part 24 of thedevice, the frame part 24 carrying two mass elements 25 and is allowedto oscillate relatively to the frame part of the nacelle. The other endof the springs extend as pairs 16, 17 and 18, 19 in opposite directionsand the springs 16-19 are pre-tensioned with a compression force whenmounted to the frame part 24 of the device. The frame part 24 of thedevice is supported by means of four wheels 26 on a rail part 27 restingon the frame part of the nacelle. The rail parts are covered with piecesof rubber 28 (preferably Volculan) on which the wheels 26 rest and thedeformation of the rubber 28 during movement of the wheels 26 on therail parts 27 contributes to the damping of the oscillations.

What is claimed is:
 1. A wind turbine comprising: a foundation, a towerextending substantially vertically and being mounted at a lower end onthe foundation, a nacelle supported by an upper end of the tower, a windrotor having at least one blade arranged on a main shaft having asubstantially horizontal rotation axis and being arranged at the nacellewith the wind rotor extending from one end of the nacelle, the at leastone blade defining a rotational plane being perpendicular to therotation axis, and oscillation damping means provided at the nacelle andbeing designed for damping oscillations of the at least one blade in therotational plane of the at least one blade, wherein the oscillationdamping means are designed for damping oscillations of at least at oneof a lower frequency and an upper frequency, wherein; (a) if the lowerfrequency oscillations are dampened, the lower frequency issubstantially equal to the first eigenfreguency in the rotational planeof the at least one blade minus a frequency of rotation of the rotor, or(b) if the upper frequency oscillations are dampened, the upperfrequency is substantially equal to the first eigenfrequency in therotational plane of the at least one blade plus the frequency ofrotation of the rotor.
 2. The wind turbine according to claim 1, whereinthe oscillation damping means are arranged inside the nacelle.
 3. Thewind turbine according to claim 1, wherein the oscillation damping meansare arranged outside the nacelle.
 4. The wind turbine according to claim1, wherein the oscillation damping means are arranged at the end of thenacelle being opposite to the end from which the rotor extends.
 5. Thewind turbine according to claim 1, wherein the oscillation damping meansare capable of damping oscillations being substantially horizontal andsubstantially perpendicular to the rotation axis.
 6. The wind turbineaccording to claim 1, wherein the oscillation damping means are designedfor damping oscillations of both the lower frequency and the upperfrequency.
 7. The wind turbine according to claim 1, wherein theoscillation damping means comprises first damping means designed fordamping oscillations of a frequency being substantially equal to thefirst eigenfrequency in the rotational plane of the at least one blademinus the frequency of rotation of the rotor and second damping meansconnected to an oscillating mass element of the first damping means andbeing designed for damping oscillations of a frequency beingsubstantially equal to the first eigenfrequency in the rotational planeof the at least one blade plus the frequency of rotation of the rotor.8. The wind turbine according to claim 1, wherein the oscillationdamping means are designed for damping oscillations of variablefrequency and comprise means for varying said frequency at leastaccording to variations of the frequency of rotation of the rotor. 9.The wind turbine according to claim 1, wherein the oscillation dampingmeans are designed for damping oscillations of a frequency deviatingless than 0.2 Hz from the first eigenfrequency in the rotational planeof the at least one blade minus, respectively plus, the frequency ofrotation of the rotor.
 10. The wind turbine according to claim 9,wherein the oscillation damping means are designed for dampingoscillations of a frequency deviating less than 0.1 Hz from the firsteigenfrequency in the rotational plane of the at least one blade minus,respectively plus, the frequency of rotation of the rotor.
 11. The windturbine according to claim 9, wherein the oscillation damping means aredesigned for damping oscillations of a frequency deviating less than0.03 Hz from the first eigenfrequency in the rotational plane of the atleast one blade minus, respectively plus, the frequency of rotation ofthe rotor.
 12. The wind turbine according to claim 1, wherein theoscillation damping means are designed for damping oscillations of afrequency in the range of 0.5-4 Hz.
 13. The wind turbine according toclaim 12, wherein the oscillation damping means are designed for dampingoscillations of a frequency in the range of 1-3 Hz.
 14. The wind turbineaccording to claim 1, wherein the oscillation damping means are designedfor damping oscillations of the at least one blade in the rotationalplane with a damping of a magnitude being equivalent to a logarithmicdecrement of oscillations of the first eigenfrequency in the at leastone blade comprised within the rotor of preferably at least 1.5%. 15.The wind turbine according to claim 14, wherein the oscillation dampingmeans are designed for damping oscillations of the at least one blade inthe rotational plane with a damping of a magnitude being equivalent to alogarithmic decrement of oscillations of the first eigenfrequency in theat least one blade comprised within the rotor of preferably at least2.5%.
 16. The wind turbine according to claim 1, wherein the oscillationdamping means are capable of damping oscillations in more than onedirection.
 17. The wind turbine according to claim 16, wherein theoscillation damping means are capable of damping oscillations in morethan one direction simultaneously.
 18. The wind turbine according toclaim 16, wherein the oscillation damping means are constructed in asymmetric or asymmetric configuration, exhibiting different or identicalresonance frequencies in two or more different directions.
 19. Themethod of damping oscillations at least in a rotational plane of a windturbine rotor comprising at least one blade by means of applyingoscillation damping means at the nacelle according to claim
 1. 20. Thewind turbine according to claim 1, wherein the wind turbine comprises anoscillation sensor for detecting oscillations of the rotor and producingan output accordingly, a control unit for receiving the output from theoscillation sensor and having means for determining, by use of theoutput, whether a predefined safety condition is fulfilled and means forregulating the operation of the wind turbine in case the safetycondition is fulfilled, and data storage means for storing data beingsignificant of the output at least a time period previous to a moment offulfillment of said condition.
 21. The method of evaluating theperformance of the oscillation damping means of a wind turbine accordingto claim 20 by outputting the stored data being significant of theoutput of the sensor for a time period immediately previous to themoment where a condition was fulfilled and analyzing said data.
 22. Thewind turbine according to claim 1, wherein the oscillation damping meanscomprise at least one mass element (M) and/or at least one springelement (S) and/or at least one damper element (D).
 23. The wind turbineaccording to claim 22, wherein the oscillation damping means comprise adirectional movable mass element (m1) connected to the nacelle by ahelical spring element (s1) and a shock absorber (d1).
 24. The windturbine according to claim 22, wherein the oscillation damping meanscomprise a pendulous mass element (m2) pivotally mounted in one end ofone or more arms, the other end of said at least one arm being pivotallymounted to a frame, where the assembly points between the arms and theframe and/or the mass element (m2) are provided with resilient elements(ds2).
 25. The wind turbine according to claim 22, wherein theoscillation damping means comprise a mass element (m3) supported by oneor more resilient columns (ds3), preferably consisting of one or moresteel members combined with one or more members made of plastic, rubberor the like in a layer construction, such as a sandwich construction.26. The wind turbine according to claim 22, wherein the oscillationdamping means comprise the mass element (m4) arranged as a pendulumconnected to an input shaft of a gearbox having an output shaftconnected to a hydro-coupling (d4).
 27. The wind turbine according toclaim 22, wherein the oscillation damping means comprise an elongatedmass element (m5) of a magnetic material arranged inside a coilelectrically connected to a resistor, said mass element (m5) beingmounted to the nacelle, preferably over a helical spring (s5).
 28. Thewind turbine according to claim 22, wherein the oscillation dampingmeans comprise a liquid container mounted to the nacelle, where theliquid (mds6) in said container constitutes the mass element, the damperelement and the spring element.
 29. The wind turbine according to claim22, wherein the oscillation damping means comprise a pendulous masselement (m7) suspended in a suspension member (ds7), such as a rubberpipe.
 30. The wind turbine according to claim 22, wherein theoscillation damping means comprise a container with a concave bottomsurface provided with one or more toothed bars that are engagement withat least one gear wheel (s8) cylindrical mass element (m8) that can rollalong the bottom surface, said container being at least partly filledwith a damping material (d8), such as a liquid.
 31. The wind turbineaccording to claim 22, wherein the oscillation damping means comprise atleast one helical spring connected to the nacelle at a first end andconnected to a mass element at the other end, the mass element beingsupported vertically by at least one wheel member which by its rimengages a damper element being a resilient material.
 32. The windturbine according to claim 30, wherein the oscillation damping meanscomprise at least two helical springs connected to the nacelle each at afirst end and extending in substantially opposite directions from thefirst ends, the other end of each spring being connected to a commonmass element so that the two springs are in a pre-tensioned state.